In life, the attempt at enjoyment is an occasion for laughter. At dawn, the dew apparently feels enjoyment, but it is as brief as the interval between the time when laughter wells up and when it ceases. There is also an affinity with the smile of the dawn.

It's my opinion that the dawn and the dew should be taken together and considered to be a symbol of the dew's attempt at happiness at dawn. Asi has taken the two to be separate. The dawn is itself an attempt at happiness, and the dew an attempt at happiness. He has used for both the similitude of a smile. The dawn is a smile in itself; since the dew is like pearly teeth, it too is a smile. (381)

FWP:

Gyan Chand points out the imagery of the verse: the 'crack of dawn' is like a smile, while the dewdrops can be seen as pearly teeth that are revealed in a 'teeth-baring smile'. For more on these images, see {67,1}.

The efforts made by humans-- or indeed, by 'existence' itself-- at enjoyment and conviviality are an occasion for sarcastic laughter. Such laughter could be that of an outside observer, or even that of the participants themselves, in the course of their doomed attempts at pleasure.

The second line appears as perhaps an echo, an illustration, or an explanation of the first line. There could be other relationships as well; in an 'A,B' verse like this, it's left entirely up to us to decide. In fact the first line is so broad and abstract that it's not even clear how to read the 'transitivity' of the second line. On the first reading (2a), we closely observe the dawn and dew, and realize that they form a perfect picture of the interval available for human pleasure. Not only is the dawn the death-knell of the dew, but both dawn and dew are imagined in the ghazal world as having the form of a smile. Thus since the first line is formally about 'existence' itself, the dawn and dew could also be imagined as (briefly) shaping smiles at their own predicament.

And on the second reading (2b), we humans are looking for a measuring rod to pin down the interval available for enjoyment and sociability, and we eventually find the ideal one: the amount of time for which the dew survives at dawn. The fact that 'dew' is literally 'night-wetness' of course adds an additional poignant touch.